There is an ever increasing need for communications with satellites, including reception of satellite broadcasts such as television and data and transmission to satellites in vehicles such as trains, cars, SUVs etc. that are fitted with one or more receivers and/or transmitters, not only when the vehicle is stationary (such as during parking) but also when it is moving.
The known antenna systems for use for mobile Direct Broadcast Satellite (DBS) reception can be generally divided into several main types. One type utilizes a reflector or lens antenna with fully mechanical steering. Another type uses phased array antennas comprised of a plurality of radiating elements. The mechanically steerable reflector antenna has a relatively large volume and height, which, when enclosed in the necessary protective radome for mobile use, is too large and undesirable for some mobile applications, especially for ground vehicles. For use with in-motion applications, the antenna housing as a whole should be constrained to a relatively low height profile when mounted on a vehicle.
The array type comprises at least three sub-groups depending on the antenna beam steering means—fully electronic (such as the one disclosed in U.S. Pat. No. 5,886,671 Riemer et al.); fully mechanical; and combined electronic and mechanical steering. The present invention relates to the last two sub-groups.
Phased array antennas are built from a certain number of radiating elements displaced in planar or conformal lattice arrangement with suitable shape and size. They typically take the form of conformal or flat panels that utilize the available space more efficiently than reflector solutions and therefore can provide a lower height profile. In certain cases the mentioned panel arrangements can be divided into two or more smaller panels in order to reduce further the height, thereby rendering such arrangements more suitable for vehicles. Such an antenna for DBS receiving is described in A MOBILE 12 GHZ DBS TELEVISION RECEIVING SYSTEM, authored by Yasuhiro Ito and Shigeru Yamazaki in “IEEE Transactions on Broadcasting,” Vol. 35, No. 1, March 1989 (hereinafter “the Ito et al. publication”). As readily shown in FIG. 1 (taken from the Ito et al. publication), the antenna consists of two antenna panels (11 and 12) that represent phased array antennas, pointed to a certain direction. During the satellite tracking they are rotated around their transverse axis (13 and 14, respectively) in order to track the satellite in the elevation plane and continuously all of them together are rotated around the axis that is perpendicular to a common platform (15) in order to track the satellite in the azimuth plane. During this movement, the antenna panels acquire different angular displacements as the angle of elevation is changed. Notwithstanding the fact that the panels 11 and 12 are angularly displaced with respect to each other, their respective axes (13 and 14) are maintained at a fixed distance with respect to each other.
As shown in FIG. 2A, at low elevation (say e1 20), the panels 21 and 22 are seen as a continuous aperture (a1 and a2) as viewed from the observation angle of the satellite 23, thereby maintaining high performance. When increasing the elevation (for example e2>e1 24 in FIG. 2B), the antenna arrangements keep being perpendicular to the observation angle of the satellite (25), but certain space between them becomes visible, thus forming certain gap g1 (26) between the projected apertures a1 and a2. Generally this is a disadvantage because it increases the average level of the sidelobes of the radiation pattern of the antenna system. The increased sidelobes result in decrease in gain and increase of the noise temperature of the antenna system and increased sensitivity to interference, thereby adversely affecting its performance.
There is thus a need in the art to provide a mobile antenna system with low profile and better radiation pattern keeping relatively low cost, suitable for mounting on moving platforms where the size is an issue as is the case in RVs trains, SUVs, bus, boats etc.